This invention relates generally to switching dc power supplies, known also as dc-to-dc converters or switching regulator, and more particularly to such a power supply capable of providing a constant dc output voltage by negative feedback. Still more particularly, the invention pertains to a dual-mode switching power supply wherein the voltage regulator switch for holding the output voltage constant is driven (i.e. turned on and off) continuously under normal load and at intervals under light load, with a view to reduction of switching loss.
The switching power supply of the type herein under consideration generally comprises a transformer having a primary winding connected to a pair of input terminals between which in turn are to be connected to a dc power supply, and a secondary winding connected via a rectifying and smoothing circuit to a pair of output terminals to be connected to a load. Connected in series with the transformer primary, the voltage regulator switch such as a field-effect transistor is driven by a switch control circuit according to the dc output voltage and the magnitude of a current flowing through the switch itself.
One of the problems encountered with the switching power supply outlined above is the power loss, known as switching loss, caused by the on-off operation of the voltage regulator switch. Switching loss must be avoided for higher efficiency of the power supply, particularly when the power requirement of the load is low. Should the voltage regulator switch be driven under light load at a switching frequency that is the same as, or higher than, that under normal load, the resulting switching loss would be too high for the power being consumed.
It has been suggested to overcome this demerit of the switching power supply by driving the voltage regulator switch continuously under normal load (normal load mode) and at intervals under light load (light load mode). Japanese Unexamined Patent Publication Nos. 2002-51549 and 2002-171760 are hereby cited as teaching such dual-mode driving of the voltage regulator switch. The light load mode is intended to make the number of switchings drastically less per unit length of time than when the switch is driven continuously in normal load mode. The reduction of switchings per unit length of time in light load mode is tantamount to the reduction of switching loss and hence to a higher overall efficiency of the converter. A loss in the stability of the output voltage during operation in light load mode is condoned as negligible compared to the merit gained.
Such dual-mode driving of the voltage regulator switch has had some problems left unresolved, however. One of the problems arises from the fact that the switching frequency is fixed. The fixed-frequency driving of the voltage regulator switch leads to the fact that the noise produced by the switching is nearly wholly contained in some specific frequency band, adversely affecting the internal circuits of the power supply or even those external thereto.
Another problem manifests itself in connection with the measurement of the load magnitude as a prerequisite for driving the voltage regulator switch in either normal or light load mode. The two unexamined Japanese patent publications cited above both rely upon the feedback signal indicative of the output voltage being applied to the load. This practice would pose no problem at all if the dc input voltage of the switching power supply were constant. Actually, it is not. It must also be taken into account that the dual-mode power supply is put to use in both 100- and 200-volt environment.
Generally, in dc-to-dc converters designed to hold the output voltage constant, the voltage level of the feedback signal changes in inverse proportion to the input voltage. The feedback signal does not, therefore, necessarily provide an almighty measure of the load magnitude.